This invention relates to the preparation of diadduct products formed from the reaction of a Diels-Alder dienophile and delta-3-carene, and more specifically, to the high yield preparation of maleic anhydride diadducts of delta-3-carene. The invention also relates to the use of the diadducts as curing agents for epoxy resin systems. The cured resins are useful in the manufacture of electronic components.
The class of compounds known as terpenes are usually defined as a variety of unsaturated C.sub.10 H.sub.16 compounds, either cyclic or acyclic, which are based on the isoprene unit. A conjugated terpene contains two carbon-carbon double bonds, separated by a single bond. Delta-3-carene is an unconjugated, monounsaturated, bicyclic terpene. It is well known that Diels-Alder dienophile monoadducts of terpenes, which are herein defined as molecules that result upon reaction of one molecule of a Diels-Alder dienophile and one molecule of a terpene, can be prepared simply by contacting the reactants at elevated temperatures. Minor amounts of diadducts, which are herein defined as molecules that result upon reaction of two molecules of a Diels-Alder dienophile and one molecule of a terpene, can also form upon contact at elevated temperatures. Not all terpenes can react to form a diadduct product however, for example, alpha-terpinene only forms a monoadduct product, and therefore, accurate predictions as to whether a terpene will produce a diadduct product cannot be made.
Although the reaction of terpenes with maleic anhydride, a typical Diels-Alder dienophile, has been reported, for example in U.S. Pat. No. 1,993,031 issued to Peterson, the particular use of delta-3-carene to produce a diadduct has not been reported.
Prior experimentation with delta-3-carene and maleic anhydride was conducted by Sugathan and Verghese and reported in "Studies in Terpenes. Part XI. A Contribution to the Understanding of the Diels-Alder Reaction of 3-Carene with Maleic Anhydride", K. K. Sugathan and J. Verghese, Journal Indian Chem. Soc.. Vol. 40. No. 2 (1963). Sugathan and Verghese reported an experiment in which maleic anhydride was reacted with delta-3-carene in a 2:1 molar ratio in the presence of acetone, leading to a residue product which contained an adduct product, but no diadduct product was reported. They also reported an experiment using a 1:1 molar ratio at 165.degree.-265.degree. C., in a solventless reaction, again with no diadduct reportedly formed. The article reports that delta-3-carene, when reacted with maleic anhydride, furnishes only a mixture of monoadduct products. The article illustrates the perception in the art that delta-3-carene exhibits only minor reactivity with maleic anhydride, and that delta-3-carene would not react with maleic anhydride to produce diadduct product.
U.S. Pat. No. 4,019,921 to Koebner discloses a condensation product of the reaction of the unsaponifiable constituents of tall oil and maleic anhydride. The tall oil is described as having a low boiling range (60.degree.-145.degree. C.) cut containing the compound carene along with other known constituents identified as alpha- and beta- pinenes, camphene, and terpineol. This low boiling cut accounted for 13% of the entire tall oil mixture. Koebner did not report whether any diadducts were produced in his reaction process.
The prior art has studied the reactions between other terpenes and maleic anhydride to increase the yield of diadduct products, but there is scarce coverage of the reactions involving delta-3-carene. Higher yields of the diadduct products are desirable since the process of separating a diadduct from its corresponding monoadduct is expensive and time-consuming.
U.S. Pat. No. 4,107,420, to Schluenz et al, discloses a process for reacting non-conjugated monocyclic terpenes and maleic anhydride in equimolar amounts, preferably at temperatures between 140.degree. C. and 200.degree. C., in the presence of iodine. Schluenz et al claim that a diadduct product in yields of greater than 15% can be achieved, and the examples disclose a product of 45% diadduct being formed. However, the reference does not identify delta-3-carene as a possible terpene compound for use in the Schluenz et al process.
It is an object of this invention to provide diadducts of delta-3-carene and various Diels-Alder dienophiles.
It is a further object of this invention to provide diadducts of delta-3-carene and a Diels-Alder dienophile in relatively high yield relative to the formation of monoadducts.
It is yet a further object of this invention to provide a cured epoxy resin, which is the curing-reaction product of an epoxy compound and diadducts of delta-3-carene and an anhydride which is reactive in a Diels-Alder dienophile manner, having a high glass transition temperature.